Light-emitting diodes (LED) lighting systems, emitting either white light or colored light, are used for numerous applications such as interior and exterior lighting, decorative lighting, entertainment and the like. LED lighting systems are typically composed of a plurality of individual LED emitters each having a different narrow spectral bandwidth. The light output of the overall system is a colorimetric combination of the light generated by the individual emitters.
The use of LED-based systems for lighting applications provides several advantages. A major advantage is the superior power conversion efficiency of LED emitters, which can reach close to 200 lumens per watt—by comparison, a typical incandescent lamp outputs only around 17 lumens per watt while a fluorescent lamp provides around 80 lumens per watt. Other advantages of LED emitters include their long lifetime, achieving around 100 000 hours of lighting, and the ability to precisely control the color of the output light. All these advantages make LED lighting systems very attractive lighting solutions.
There are, however, a few difficulties related to LED-based lighting. Firstly, the spectrum of individual LED emitters tends to shift over time, resulting in an ageing-related drift of the output color. This shift of the output color evolves in a complex manner and cannot be predicted by theoretical models. Secondly, the output color can vary as a function of operation conditions such as the temperature of the LED lighting system or the currents that drive one or more of the LED emitters.
The quality of the light generated by a LED lighting system affects the perceived colors of an illuminated scene: the color rendering property of a LED system is therefore a factor to be taken into account. Color rendering can be characterized using the CRI (Color Rendering Index), which is a color rendering metric standardized by the CIE (Commission Internationale de l'Éclairage), or the CQS (Color Quality Scale), which is an alternative metric proposed by the NIST (National Institute of Standards and Technology). For example, it is recognized in the literature that a CRI of at least 90 is desirable for lighting applications.
Color rendering metrics are particularly meaningful for LED lighting systems that generate white light. A minimum of three primary colors are required for additive color synthesis of white light, typically red, green and blue (RGB). Typical LED lighting systems with only three LED emitters cannot easily provide white light with good color rendering properties. LED-based lighting systems having four or more LED emitters with different “primary” colors can be used to reach or to exceed the CRI threshold of 90, if appropriately controlled. At least four LED emitters are therefore preferred for quality lighting applications, such as in museums and for advertisement purposes. However, to obtain the desired results the LED light system must be carefully controlled to achieve a constant color output for all values of operation temperature and drive current, for the lifetime of the LED lighting system.
It is known in the art to use color feedback during operation of a LED lighting system in order to compensate for drifts of the output color. A color detector is used to “read” the color of the output light of the LED system and a correction of the operating conditions can be applied to compensate for any shift that arises, based on the information obtained. The main difficulty associated with implementing a color feedback scheme is the measurement of the color of the LED emitters. The CIE introduced three functions to represent the three color receptors in an average human eye. The color of the light emitted from a lighting system can be represented by three quantities (X, Y, Z) that represent the integration of a measured spectrum of the light over these functions. A direct manner of obtaining the color of a light source is therefore to measure the spectrum of its emitted light and then integrate over the CIE functions to obtain the three quantities. However, measuring the spectrum of a lighting system requires the use of expensive equipment such as a spectrometer. An alternative is to use a color sensor or colorimeter. Such sensors are typically composed of three filtered detectors and are much more affordable than spectrometers. The filters can be selected to match the three colorimetric functions of the CIE, in which the sensor output corresponds to the CIE color coordinates X, Y and Z of the detected light. In practice, it is however very difficult to obtain filters that correspond exactly to the colorimetric functions. Therefore, the values outputted by color sensors do not really correspond to the (X,Y,Z) quantities characterizing the color of the output light.
There therefore remains a need for a control method for a LED lighting system that alleviates at least some of the drawbacks above.